This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ligand K-edge XAS provides a direct probe of ligand metal bonding. We have developed this methodology to investigate the electronic structures of model complexes and protein active-sites of a number of clusters and sites, in particular Fe-S, Cu-S clusters and dithiolene-containing Mo/W sites. In the previous proposal period, among several studies, we evaluated the generality of the difference between HiPIPs and ferredoxins and systematically studied the effect of H-bonding, solvent interaction and effect of changing dielectric field around these and other clusters using well-characterized model complexes and proteins, and combined experimental results with DFT calculation. We have also developed the bonding/geometric structure correlations in the Mo(tris)dithiolenes and the mechanism of oxo trasnsfer in the DMSO reductase family of enzymes and models. In this proposal we plan to extend this methodology, and combine it with complementary spectroscopies, to study several Fe-S systems, such as the interaction of SAM with the Fe4S4 cluster in pyruvate lyase activating enzyme;MutY - a DNA glycosylase from Escherichia coli;Fe2S2 protein binding to ?9 desaturase;and effects of Cys->Ser mutations on electron delocalization in iron sulfur clusters. We also intend to study a series of Mo mono- and bis-oxo bis(dithiolene) complexes that model the states of the sulfite oxidase family. The specific aim is to define the electronic structure of enzymes and model complexes and understand the correlations between structure and function of enzymatic reactions.